The principal area of focus for this project is the identification of heritable characteristics of ion transport which may distinguish individuals with a primary affective disorder from normal controls. The present characteristic under study is the distribution of lithium ions across the membrane of the human erythrocytes in in vivo conditions. A study of twenty-five patients with major affective illness found that the intraerythrocyte to plasma (RBC/plasma) ratio of lithium ion concentration did not discriminate between unipolar and bipolar patients with major affective illness. A racial difference, with black patients having a higher ratio than white patients, was confirmed.